Based on the VSM data, it appears that the 20nm Fe3O4 nanoparticle is paramagnetic while the 18nm Fe3O4 nanoparticles are superparamagnetic. To understand this further, it is important to note that the magnetization of a material increases with decreasing size. This means that even though the 18nm Fe3O4 nanoparticles are smaller than the 20nm Fe3O4 nanoparticles, they show higher magnetization and therefore represent the superparamagnetic behavior. When you hold a magnet to the powdered Fe3O4 nanoparticles, the higher magnetization caused by the small size of the particles leads to the strong magnetic attraction that you observe.

Ref:

1. https://doi.org/10.1007/s42452-019-0244-7

2. https://doi.org/10.1007/s42250-020-00153-4

3. https://doi.org/10.1016/j.cplett.2020.137944

4. https://doi.org/10.1007/s00339-021-04653-z

5. https://doi.org/10.1007/s40089-021-00343-z

6. https://doi.org/10.1007/s42250-022-00438-w

7. https://doi.org/10.14233/ajchem.2022.23840

8. https://doi.org/10.1007/s42250-022-00570-7

9. https://doi.org/10.1007/s00339-022-06370-7

10. https://doi.org/10.1007/s42250-023-00615-5

Thank you very much for your reply @R.sagayaraj. I will read your articles with interest.

It seems to me that your nanoparticles formed an aggregates because Hc value is high for paramagnetic material. Maybe when aggreagates are formed the response of ensemble of nanoparticles is ferrimagnetic like. To be sure if these powders superparamgnetic or not zero field colling (ZFC) or field cooling (FC) measurement can be done.

Thank you for your valuable comment @rafael shakirzyanov.

Aggregation is seen in S-TEM and Fe-SEM results. Is it due to aggregation that the hc value of the 18 nm nanoparticle is 95, while the hc value of the 20 nm nanoparticle is 63?

We determined that both nanoparticles were in Fe304 structure, in accordance with the Xrd results of the literature. Is there an article you can recommend that explains this situation?

Can I make an aggregation comment to explain the relationship between the hc and ms value of these data?

@rafaelshakirzyanov I don't have the means to do the analysis you mentioned. therefore, I need to explain that the data is superparamagnetic, supporting it with the literature. If there are articles or studies that you can suggest, I would appreciate it if you could help with this issue.

Can anyone help me to interpret this data ? What unit of magnetic momen should be at Y axis?

With reference

thanks in advance

Dear Zeynep,

If you are looking for better proof or evidence of your superparamagnetic nanoparticles, you need to take first-order reversal curve (FORC) analysis. We perform this analysis for researchers affordably. Please contact me if you need more information. Email: [email protected]

Regards

Perhaps I did not understand something, but it seems to me that the opinion of the respected R. Sagayaraj is noticeably at odds with mine.

Firstly, Zeynep Kaya Özcan did not say that the first sample is attracted to the magnet worse than the second, and this in itself already testifies in favor of the fact that the material in both cases as a whole remains ferrimagnetic, which is what magnetite (or related maghemite (g-Fe2O3) should be, if in in the sample, as it often happens, both of these phases coexist). If it were hematite (a-Fe2O3), the attraction would not be very noticeable.

Secondly, in both cases we see large and comparable values of magnetization, which are quite close to the value of 92 emu/g inherent in bulk magnetite samples.

Thirdly, contrary to the statement of R. Sagayaraj, in the data provided by Zeynep Kaya Özcan, we see a decrease in magnetization with a decrease in size, which is precisely a characteristic property of nanoparticles. This can happen for various reasons: 1) due to the increase in the contribution of surface cations with broken exchange bonds, 2) due to the increased influence of quantum fluctuations (due to the uncertainty principle, primarily for extremely small particles), 3) due to the oxidation of the surface and the transition of part of the magnetite to maghemite with a lower magnetization, and further in due to the instability of this phase – in hematite. There are other explanations.

When the dimension decreases, of course, the Curie point also decreases. But it is not so large that ferrimagnetic magnetite becomes paramagnetic at room temperature. In addition, paramagnetic materials do not give such a nonlinear hysteresis curve with such a high magnetization.

Even the lowest value of the critical size for the transition to a superparamagnetic state, given in the literature, as far as I remember, is 22 nm for magnetite (if necessary, I can look for references), so particles of 18-20 nm should exhibit superparamagnetic behavior.

As for the increased value of coercivity, I fully share the opinion (assumption) of Rafael Shakirzyanov about the formation of particle conglomerates.

About aggregation: the Hc value is influenced not only by the size, but also by the shape of aggregates, and the connections between the particles in the aggregate, therefore, as I believe, it is difficult to identify some clear relationship and deduce a pattern suitable for each case.